Solid propellant dual pulse rocket motor loaded case and ignition system and method of manufacture

ABSTRACT

A solid propellant dual pulse rocket motor has a pressure vessel containing two pulse grains separated by a barrier insulator. An igniter assembly disposed at a fore end of the pressure vessel selectively ignites a first pulse grain from a central channel within the grain. The igniter assembly also ignites a second pulse grain by ejecting hot combustion gases onto the fore end of the grain. Using such an ignition arrangement, a dual pulse rocket motor may be constructed using standard off-the-shelf ignition components.

This application is a divisional of application Ser. No. 08/469,759,filed Jun. 6, 1995 now U.S. Pat. No. 5,600,946, which is a continuationof Ser. No. 08/235,308, filed Apr. 29, 1994, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to rockets and other self-propelledmissiles and projectiles, and more specifically, to solid propellantrocket motors for such devices which are capable of providing multiplepropulsive impulses to the vehicle in which they are installed.

2. Description of the Related Art

Motors for rockets, other types of propelled missiles and the likegenerally are of two types. The first, the liquid propellant motor, hasa tank containing a liquid fuel such as liquid hydrogen or ammonia, anda tank containing a liquid oxidizer such as liquid oxygen, nitric acidor fluorine. The two liquids are mixed together in a combustion chamberin a specific proportion and at flow rates designed to cause the liquidto spontaneously combust. The combustion products are expelled from therocket's exhaust nozzle, thereby providing a thrusting force to propelthe rocket. Liquid propellant motors are useful for their ability to beprecisely controlled; stopped and restarted; and checked out, fired andcalibrated before actual use. Liquid propellant motors also areadvantageous in that they provide a wide range of specific impulseratings, i.e., the amount of thrust per unit mass of fuel burned perunit time; and a relatively long burn time.

The other major type of rocket motor is the solid propellant motor. Insolid propellant systems, the rocket is propelled by a solid fuel chargeor "grain" that initially is ignited by an electric or pyrotechnicigniting device. As the grain burns, it generates exhaust gases andother combustion products which are expelled through a nozzle at the endof the rocket. The combustion products are expelled from the rocket'sexhaust nozzle, thereby providing a thrusting force to propel therocket. The advantages of the solid propellant motor are its relativelysimple structural design and its ease of use.

In many applications, it is desirable to use a solid propellant motorthat can provide two separate and distinct propulsive impulses, i.e., adual pulse motor. For example, the first pulse in a dual pulse motorcould be used to fire a missile towards its target. When the missile isnear the target, the second pulse could be fired to accelerate themissile, increase its force on impact with the target, and enhance thedamage imparted to the target.

Several types of dual pulse motors have been developed. For example,U.S. Pat. No. 4,936,092 to Andrew discloses a system in which grains forthe different pulses may be contained in separate, detachable stages.When the grain for one pulse has been entirely consumed, its stage maybe jettisoned and a new stage ignited. This arrangement, however,entails the duplication of relatively complicated mechanical parts, thecoordination of operations therebetween, and additional weight andmanufacturing considerations.

Also, as shown in U.S. Pat. No. 3,122,884 to Grover et al., the grainsmay be contained in separate combustion chambers within a single stage,where the chambers have separate nozzles or share a common nozzle. Sincethis scheme still entails some duplication of parts, the propellant loadthat can be accommodated in the motor is necessarily limited and thecost and weight of the motor are increased. Also, the multiple nozzleconfiguration limits the size of each nozzle, thereby decreasing theavailable specific impulse available from the motor.

An alternative to these designs is shown in U.S. Pat. No. 3,908,364 toLeFebvre et al. and U.S. Pat. No. 4,085,584 to Jones et al. In thesesystems, the grains for each pulse may be accommodated in a singlecombustion chamber. To prevent the second grain from igniting oncecombustion has begun in the first grain, the grains are separated by athin thermal insulation membrane at their interface. This membraneprotects the second grain from inadvertent ignition while the firstgrain is burning. Once the first grain is spent, a separate igniterinitiates combustion of the second grain to begin the second pulse,thereby destroying the membrane and permitting combustion products toexit from the nozzle.

While the above-described prior art systems serve their purpose, theyrequire specialized ignition systems to accommodate the uniquecombustion characteristics of dual pulse motors. Also, the specializedignition characteristics of dual pulse motors are fraught with uniqueproblems such as tolerance stack-up and the difficulty of conductingreliable inspections and assemblies.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a solid propellant dualpulse motor that can use standard solid propellant motor ignition traincomponents such as through-bulkhead-initiators (TBIs), explosivetransfer assemblies (ETAs) and safe-and-arms (S&As).

It is a further object of the present invention to provide a solidpropellant dual pulse motor that which is relatively easy to inspect andassemble.

It is yet another object of this invention to provide a solid propellantdual pulse motor that is substantially free from tolerance stack-upconsiderations and the like.

It is still another object of the present invention to provide a methodof manufacturing a solid propellant dual pulse motor that can beperformed easily, reliably and inexpensively.

The above objects are achieved by providing a solid propellant dualpulse rocket motor that has a pressure vessel containing two pulsegrains separated by a barrier insulator. An igniter assembly disposed ata fore end of the pressure vessel selectively ignites a first pulsegrain from a central channel within the grain. The igniter assembly alsoignites a second pulse grain by ejecting hot combustion gases onto thefore end of the grain. Using such an ignition arrangement, a dual pulserocket motor may be constructed using standard off-the-shelf ignitioncomponents.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of this invention will becomeapparent and more readily appreciated from the following description ofthe presently preferred exemplary embodiments, taken in conjunction withthe accompanying drawings, of which:

FIG. 1 is a cross-sectional diagram of a case assembly according to thepresent invention;

FIG. 2 is a cross-sectional diagram of a solid propellant two pulserocket motor according to the present invention;

FIG. 3 is an enlarged cross-sectional diagram of a dual pulse igniterassembly according to the present invention;

FIG. 4 is a cross-sectional diagram of a casting assembly for forming aPhase 2 grain according to the present invention; and

FIG. 5 is a cross-sectional diagrain of a casting assembly for forming aPhase 1 grain according to the present invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EXEMPLARY EMBODIMENTS

The foundation of the present invention is the motor case assembly 10shown in FIG. 1. The motor case assembly 10 has a pressure vessel 12disposed within a skirt 14. Preferably, the pressure vessel 12 has a12.24 inch maximum outer diameter and is made from a filament-woundgraphite-epoxy resin composition.

The skirt 14 preferably has a 12.24 inch diameter and is made from afilament-wound graphite-epoxy resin composition. Also, to alleviate anydifferential strains between the pressure vessel 12 and the skirt 14, itis advantageous to provide adhesive shear plies at the interface betweenthese two components.

It should be noted that FIG. 1 and all other Figures are cross-sectionallongitudinal views of the present invention and that, although notshown, the invention and parts contained therein generally have acircular cross-section when viewed axially.

The fore end of the skirt 14 has a fore end ring 16 attached thereto,and the aft end of the skirt 14 has an aft end ring 18 attached thereto.The fore end ring 16 provides for transfer of thrust loads and formating to motor processing tooling, while the aft and ring 18 supportstwo nozzle actuators 40 (shown in FIG. 2) and also mates to motorprocessing tooling. Preferably, the fore ring 16 and aft ring 18 aremade from an aluminum alloy and are bonded and riveted to the skirt 14;however, other equivalent suitable materials and bonding techniques willbe readily apparent to those skilled in the art.

The pressure vessel 12 has a fore polar boss 20 and an aft polar boss 22at fore and aft ends, respectively, along its longitudinal axis. Thefore polar boss 20 provides for mating and support of the motor igniterassembly 34 (shown in FIG. 2) as described in more detail below, and theaft polar boss 22 mates with the nozzle assembly 38 (also shown in FIG.2) to support blowout loads. Preferably, the fore polar boss 20 and theaft polar boss 20 each are made from a titanium alloy or other compoundhaving good high temperature characteristics (such as steel or aluminum)as will be apparent to those skilled in the art.

The pressure vessel 12 has a Pulse 1 grain 24 disposed in an aft portionof the interior of the pressure vessel 12 and a Pulse 2 grain 26disposed in a fore portion thereof. Preferably, the Pulse 1 grain 24 ismade from about 44 lb. of an aluminum powder-fueled, hydroxyl-terminatedpolybutadiene (HTPB) binder composition, and the Pulse 2 grain 26 ismade from about 22 lb. of a similar composition having a higher burnrate. In a preferred embodiment, the Pulse 2 grain 26 has about a 68%higher burn rate than the Pulse 1 grain 24.

A fore insulator 28 is disposed between the pressure vessel 12 and thePulse 2 grain 26 to provide erosion and thermal protection for thepressure vessel 12 and the Pulse 2 grain 26 while Pulse 1 is burning andduring the interpulse delay. Additionally, the Pulse 2 grain 26 has agrain support 30 (preferably made from a foam material) centrallydisposed at its fore end. Also, a barrier insulator 32 is disposedbetween the Pulse 1 grain 24 and the Pulse 2 grain 26 to provide similarprotection to the aft end of the Pulse 2 grain 26, and an aft insulator33 is disposed between the aft end of the Pulse 1 grain 24 and the aftend of the pressure vessel 12 for similar reasons. Preferably, the foreinsulator 28, the barrier insulator 32 and the aft insulator 33 are madefrom Kevlar-filled ethylene propylene diene monomer (EPDM) material andinclude stress-relief boots to provide propellant bondline stress reliefduring cold-temperature storage and operation.

The Pulse 1 and 2 grains 24, 26 and the fore and barrier insulators 28,32 cooperatively define a central ignition cavity 34 and, as shown inFIG. 2, a motor igniter assembly 36 is bolted to the fore polar boss 20at a fore end of the ignition cavity 34. In addition to showing theplacement of the motor igniter assembly 36 in the pressure vessel 12,this Figure also shows the flexseal thrust vector control (TVC) nozzle38 and one of its associated actuators 40.

As shown in FIG. 3, the motor igniter assembly 36 includes an igniterclosure 42 and an igniter case 44 projecting therefrom. The igniterclosure 42 fits into the fore polar boss 20 and preferably is made of atitanium alloy. The igniter case 44 has three Pulse 1 igniter grains 46,48, disposed therein. The grains 48 preferably are made from acase-bonded propellant, and the grain 46 preferably is a case-bondedbooster propellant. The grains 46, 48 are separated from ignitionpellets 50 (preferably BKNO₃ pellets) which are separated from the grain46 by a steel screen 52. When the ignition pellets 50 are ignited by,for example, two squibs or TBIs, they ignite the grains 46, 48, therebyconsuming the igniter case 44.

Preferably, the igniter case 44 is made from aluminum or an aluminumalloy; however, any other equivalent composition that is readilyconsumed during ignition, e.g., magnesium or steel, can be used, as willbe apparent to those skilled in the art.

When in place, the igniter closure 42 cooperates with the fore polarboss 20 to define a toroidal chamber 54 containing the Pulse 2 ignitergrain 56. One or more nozzle ports 58 which allow ignition gases fromthe Pulse 2 igniter grain 56 to pass from the toroidal chamber 54 to theinterior of the pressure chamber 12. Preferably, the nozzle ports 58 aremade from short lengths of silica-phenolic material.

More specifically, when the Pulse 2 grain 56 is ignited by, for example,a squib or TBI, hot gases pass from the toroidal chamber 54 through thenozzle ports 58 and impinge upon the grain support 30 to melt it. Inthis process, the gases ignite the Pulse 2 propellant grain 28 torupture the barrier insulator 32 and start the motor's second pulse.

Preferably, the motor igniter assembly includes one or more Pulse 1pressure ports 60 and one or more Pulse 2 pressure ports 62 throughwhich the pressures generated during the ignition process may bemeasured.

A method of making the above-described invention will now be describedwith reference to FIG. 4. First, the fore, barrier and aft insulators28, 32 and 33 are fabricated. Then, the fore insulator 28 is placed inPulse 2 casting tooling 64, the grain support 30 is fitted around thecentral throat of the fore insulator 28 and the Pulse 2 grain 26 is castunder vacuum to remove all air from the Pulse 2 grain area and to ensurethat the barrier insulator 32 is pushed out to its proper shape. Next,the Pulse 2 grain 26 is pressure cured.

Once the Pulse 2 grain 26 has been cured, the lid 66 is removed from thePulse 2 casting assembly 64 and the barrier insulator 32 is attached tothe Pulse 2 grain 26 and the fore insulator 28, and the fore insulator28 is trimmed back. Then, the aft insulator 33 is attached to thebarrier insulator 32, and as shown in FIG. 5, the Pulse 1 casting tool68 is attached to the Pulse 2 casting tool 64. Finally, the Pulse 1grain 24 is cast and cured to obtain a grain assembly.

Before the insulators 28, 32 and 33 are used in this process, theinterior surfaces of each are coated with an adhesive liner. Preferably,this liner uses the same binder as the propellant grains 24, 26 andincludes a carbon black filler. After two 12.5 mil thick coats of theliner are applied to the exterior of the fore and aft insulators 28 and33, the liner is cured and covered with a wash coat of isophoronediisocyanate (IPDI) to enhance bondline strength.

Once the grain assembly has been formed, graphite fiber (or any otherfiber having a sufficiently high tensile strength, such as steel, etc.)is wound around the grain assembly, coated with an epoxy resin, andcured to form the pressure vessel 12. The polar bosses 20 and 22, skirt14, motor igniter assembly 36 and TVC nozzle 38 then are attached to thepressure vessel 12.

Although a few preferred embodiments of the invention have been shownand described, it will be appreciated by those skilled in the art thatchanges may be made in these embodiments without departing from theprinciples and the spirit of the invention, the scope of which isdefined in the appended claims.

What is claimed is:
 1. A method of manufacturing a solid propellant dualphase motor, said method comprising the steps of:casting a second pulsegrain in a fore insulator; curing said second pulse grain; bonding abarrier insulator to said second pulse grain., said barrier insulatorbeing formed of a material selected such that it provides erosion andthermal protection for said second pulse grain during burning of a firstpulse grain; bonding an aft insulator to said barrier insulator; castinga first pulse grain in said aft insulator; and installing ignition meansfor selectively igniting said first pulse grain and for subsequentlyselectively igniting said second pulse grain.
 2. The method of claim 1,said second pulse grain casting step comprising a step of placing agrain support in a central fore portion of said fore insulator.
 3. Themethod of claim 1, wherein said second pulse grain casting stepcomprises a step of casting said second grain under vacuum.
 4. Themethod of claim 1, said barrier insulator bonding step comprising a stepof trimming said fore insulator.
 5. The method of claim 1 furthercomprising the steps of:winding a filament around said grain assembly;coating said filament with a resin; and curing said resin to form apressure vessel.
 6. The method of claim 5, wherein said filament is madefrom a material in the group consisting of graphite and steel.
 7. Themethod of claim 6, wherein said filament is a graphite filament.
 8. Themethod of claim 5 further comprising the steps of:coating an interiorsurface of at least one of said insulators with a liner; and curing saidliner.
 9. The method of claim 8 further comprising the step of:applyinga wash coat to said cured liner.